DAIRY CHEMISTRY AND BIOCHEMISTRY Weaning 10 0 100 Figure 1.3 Time-course of mammary development in rats(from Tucker, 1969) The substrates for milk synthesis enter the secretory cell across the basal membrane(outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped)form abasket around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, there- by causing ejection of milk from the lumen into the ducts. Development of mammary tissue commences before birth, but at birth he gland is still rudimentary. It remains rudimentary until puberty when rery significant growth occurs in some species; much less growth occurs in other species, but in all species the mammary gland is fully developed at puberty. In most species, the most rapid phase of mammary gland develop- ment occurs at pregnancy and continues through pregnancy and partur ition, to reach peak milk production at weaning. The data in Figure 1.3 the development pattern of the mammary gland in the rat, the species has been thoroughly studied in this regard Mammary development is under the regulation of a complex set of hormones. Studies ng endocrinectomy(removal of different endocrine organs) show that the principal hormones are oestrogen, progesterone growth hormone, prolactin and corticosteroids( Figure 1.4)
6 DAIRY CHEMISTRY AND BIOCHEMISTRY 3 t-” 10 0 0 100 200 Days Figure 1.3 Time-course of mammary development in rats (from Tucker, 1969). The substrates for milk synthesis enter the secretory cell across the basal membrane (outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped) form a ‘basket’ around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, thereby causing ejection of milk from the lumen into the ducts. Development of mammary tissue commences before birth, but at birth the gland is still rudimentary. It remains rudimentary until puberty when very significant growth occurs in some species; much less growth occurs in other species, but in all species the mammary gland is fully developed at puberty. In most species, the most rapid phase of mammary gland development occurs at pregnancy and continues through pregnancy and parturition, to reach peak milk production at weaning. The data in Figure 1.3 show the development pattern of the mammary gland in the rat, the species that has been thoroughly studied in this regard. Mammary development is under the regulation of a complex set of hormones. Studies involving endocrinectomy (removal of different endocrine organs) show that the principal hormones are oestrogen, progesterone, growth hormone, prolactin and corticosteroids (Figure 1.4)
PRODUCTION AND UTILIZATION OF MILK ATROPHIC GLAND Oest +GH+C DUCT GROWTH Ocst +Prog+Pl+ LOBULO-ALVEOLAR GROWTH PL +C MILK SECRETION Figure 1.4 The hormonal control of mammary development in rats. Oest, Oestrogen: Prog, progesterone; GH, growth hormone; PL, prolactin; C, corticosteroid 1.5 Ultrastructure of the secretory cell The structure of the secretory cell is essentially similar to that of other eukaryotic cells. In their normal state, the cells are roughly cubical, c. 10 um in cross-section It is estimated that there are c. 5 x 10 2 cells in the udder of the lactating cow. a diagrammatic representation of the cell is shown in igure 1.2d. It contains a large nucleus towards the base of the cell and is surrounded by a cell membrane, the plasmalemma. The cytoplasm contains the usual range of organelles mitochondria: principally involved in energy metabolism (tricarboxylic acid(Krebs) cycle) endoplasmic reticulum: located towards the base of the cell and to which are attached ribosomes, giving it a rough appearance(hence the term, rough endoplasmic reticulum, RER). Many of the biosynthetic reaction of the cell occur in the rer: Golgi apparatus: a smooth membrane system located toward the apical gion of the cell, where much of the assembly and 'packaging of synthesized material for excretion occur
PRODUCTION AND UTILIZATION OF MILK 7 ATROPHIC GLAND Ocst + GH + C DUCT GROWTH LOBULO-ALVEOLAR GROWTH MILK SECRETION Figure 1.4 The hormonal control of mammary development in rats. Oest, Oestrogen; Prog, progesterone; GH, growth hormone; PL, prolactin; C, corticosteroids. 1.5 Ultrastructure of the secretory cell The structure of the secretory cell is essentially similar to that of other eukaryotic cells. In their normal state, the cells are roughly cubical, c. 10 pm in cross-section. It is estimated that there are c. 5 x 10’’ cells in the udder of the lactating cow. A diagrammatic representation of the cell is shown in Figure 1.2d. It contains a large nucleus towards the base of the cell and is surrounded by a cell membrane, the plasmalemma. The cytoplasm contains the usual range of organelles: 0 mitochondria: principally involved in energy metabolism (tricarboxylic acid (Krebs) cycle); 0 endoplasmic reticulum: located towards the base of the cell and to which are attached ribosomes, giving it a rough appearance (hence the term, rough endoplasmic reticulum, RER). Many of the biosynthetic reactions of the cell occur in the RER; 0 Golgi apparatus: a smooth membrane system located toward the apical region of the cell, where much of the assembly and ‘packaging’ of synthesized material for excretion occur;